Optical scanning system with variable focus lens

ABSTRACT

Apparent distance of a pixel within an optical field of view is determined. Incoming light is scanned along a raster pattern to direct light for a select pixel onto a light distance detector. The distance is sampled for each pixel or for a group of pixels. The light distance detector includes a concentric set of rings sensors. The larger the spot of light corresponding to the pixel, the more rings are impinged. The diameter of the spot is proportional to the distance at which the light originated (e.g., light source or object from which light was reflected). Alternatively, a variable focus lens (VFL) adjusts focal length for a given pixel to achieve a standard spot size. The distance at which the light originated correlates to the focal length of the VFL.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation of U.S. patent application Ser. No.09/740,272 filed Dec. 18, 2000 for “Method and Apparatus for DeterminingOptical Distance” of Melville et al., which is a continuation of U.S.patent application Ser. No. 09/188,991 filed Nov. 9, 1998 for “Methodand Apparatus for Determining Optical Distance,” of Melville et al. Thecontents of such applications are incorporated herein by reference andmade a part hereof.

BACKGROUND OF THE INVENTION

[0002] This invention relates to methods and apparatus for determiningan optical distance, such as a distance of an object within a field ofview, and more particularly to a method and apparatus for scanningdistances within a field of view.

[0003] A conventional camera includes an objective lens and a lightdetector, such as a photographic film, CCD array or other photosensitivedevice or structure. Light from a viewing environment enters the camerathrough the objective lens and impinges on the light detector. Theportion of the viewing environment for which light enters is thecamera's field of view. Some cameras pass the light to a viewfinder oreyepiece allowing an operator to select a desired field of view from thebackground environment. To take a picture or record, the light detectorcaptures frames of the background light from the field of view.

[0004] Often the field of view is divided into discrete picture elementsor pixels. In conventional digital video cameras the light detectorrecords data for each pixel within the field of view for a given videoframe. The data includes color, intensity and the pixel coordinates(i.e., x,y coordinates).

[0005] Conventional still cameras and video cameras include optics forfocusing within the field of view. Thus, an operator can select to focuson a near field object or a far field object. Some cameras even includeautofocus devices which automatically adjust the focal length of theobjective lens to focus within the field of view.

SUMMARY OF THE INVENTION

[0006] According to the invention, an apparent distance of one or morepoints within an optical field of view is determined. For example, anapparent distance is determined for each pixel, or for one or more groupof pixels, within a field of view. Such distance is also referred to asa depth of view. One advantage of the invention is that pixel data foran object viewed may be recorded, input to a computer and mappedenabling display of a 3-dimensional model of the object. Anotheradvantage is that an augmented display device or camera device can havevariable accommodation.

[0007] Incoming light is scanned along a raster pattern to direct lightfor a select pixel onto a light distance detector. The distance issampled for each pixel or for a group of pixels. In one embodiment alight distance detector is used. In an alternative embodiment a variablefocus lens is used.

[0008] The light distance detector includes a concentric set of ringsensors. The larger the spot of light corresponding to the pixel, themore rings are impinged. For light entering from a far distance, such asfrom infinity to about 20 feet, the spot will be small. For light comingfrom closer distances the spot is larger. The diameter of the spot isproportional to the distance at which the light originated (e.g., lightsource or object from which light was reflected). Each ring correspondsto a distance. The number of rings impinged determines the distance forthe pixel being sampled.

[0009] The variable focus lens (VFL) is included in the light path. Fora given pixel to be sampled, the focal length of the VFL is varied toachieve a small spot size. The distance at which the light originatedcorrelates to the resulting focal length of the VFL.

[0010] Although, distance is sampled for each pixel or for a group ofpixels, light intensity and color also may be sampled to record adigital image of a field of view, such as for a camera implementation.

[0011] The invention will be better understood by reference to thefollowing detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a diagram of a conventional image detection apparatus;

[0013]FIG. 2 is a diagram of an apparatus for scanning optical distancewithin a field of view according to an embodiment of this invention;

[0014]FIG. 3 is a diagram of the light detector of FIG. 2;

[0015]FIG. 4 is a diagram of the light detector of FIG. 3 with animpinging spot of light;

[0016]FIG. 5 is a diagram of an electro-mechanically variable focus lensfor a lensing system of FIG. 2 according to an embodiment of thisinvention;

[0017]FIG. 6 is a diagram of an alternative variable focus lensembodiment for the lensing system of FIG. 2;

[0018]FIG. 7 is a diagram of another alternative variable focus lensembodiment for the lensing system of FIG. 2;

[0019]FIG. 8 is a diagram of a plurality of cascaded lens for thelensing system of FIG. 2 according to an embodiment of this invention;

[0020]FIG. 9 is a block diagram of a feedback control scheme fordetecting light distance according to an embodiment of this invention;and

[0021]FIG. 10 is a diagram of an image recording apparatus according toan embodiment of this invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

[0022] Overview

[0023] Referring to FIG. 1, in a conventional image detection apparatus10, background light from a field of view F impinges on an objectivelens 14 which converges the light toward a light detector 16. In adigital camera the light detector 16 may be a charge-coupled device(CCD), which also serves as a viewfinder. Light from objects within thefield of view F, such as a first object 18 (e.g., a tree) and a secondobject 20 (e.g., a bird) is captured to record an image of the field ofview or a part thereof.

[0024] Referring to FIG. 2, an apparatus 30 detects optical distance(i.e., depth of view) for objects 18, 20 in the field of view Faccording to an embodiment of this invention. The apparatus 30 includesan objective lens 14, a scanning system 32, a lensing system 34 and alight distance detector 36. Background light 12, from the field of viewF, including light reflected from the objects 18, 20 enters theapparatus 30 at the objective lens 14. The light is directed to thescanning system which scans the background light along two axes toselect at any given time a pixel area of the field of view to beanalyzed. Light originating within the select pixel area is directedinto the lensing system 34 which converges the light onto the lightdistance detector 36. Preferably only light originating from a single,select pixel area is focused onto the light distance detector 36. Inalternative embodiment the size of the area being measured for distancemay vary to include multiple pixels. The size of the field portionmeasured for distance is determined by the size of a mirror surface onscanners 38, 40 within the scanner system 32, the relative location ofthe mirror surface relative to the objective lens 14 and the lensingsystem 34, and the relative location of the lensing system 34 relativeto the light distance detector 36.

[0025] During operation, the scanning system 32 periodically scans alonga prescribed scanning pattern, such as a raster pattern. For scanning atwo dimensional raster pattern, a horizontal scanner 38 scans along ahorizontal axis and a vertical scanner 40 scans along a vertical axis. Asample is taken at the light distance detector for multiple points alongeach given horizontal scanning line. Such sample, for example,corresponds to a pixel. The light distance detector signal 35corresponds to the depth of view of the light sample. In someembodiments a table of correlation data is stored in memory 37. Acontroller 43 compares the light distance detector signal 35 to entriesin the table to derive the depth of view for the light sample. Thedetermined depth of view is read from the memory 37 and stored as thedepth of view for the pixel that was sampled. Thus, a distance (i.e.,depth of view) is determined for each pixel within the field of view.

[0026] In some embodiments the distance is stored in memory togetherwith the pixel coordinates (i.e., field of view coordinates) for laterretrieval. Light intensity and color also may be detected and stored, asfor a camera or other recording implementation.

[0027] Light Distance Detector

[0028] Referring to FIG. 3, a light distance detector 36 according toone embodiment of this invention includes concentrically positionedlight detection sensors 42-50 that form a set of concentric rings. Thenumber of rings and radial increment may vary depending on the distanceresolution desired. Light 52 from select pixel region is converged bythe lensing system 34 onto the light distance detector 36. Referring toFIG. 4, such light 52 forms a spot 54, preferably centered at the centerof the detector 36. The smaller the spot 54, the farther the focalsource of the light 52 for the select pixel. For light, at approximately20 feet or further from the system 30, the light waves are flat andfocus down to a common point size. Accordingly, light at such distanceis not differentiated (i.e., resolved). Light from zero feet toapproximately 20 feet from the system 30, however is differentiated byidentifying which ring sensors detect light. In the example illustratedin FIG. 4, the light spot 54 encompasses sensors 42-48. A specificdistance corresponds to activation of such sensors 42-48.

[0029] An alternative method for detecting the optical distance forpixel light is achieved, by modifying the focal length of the lensingsystem 34 until a spot of a desired standard size is achieved. Forexample, the focal length may be varied until the spot size encompassesonly sensors 42 and 44. Alternatively, only sensor 42 may define thestandard spot size or only sensors 42-46, or some other prescribedsubset of sensors 42-50 may define the prescribed spot size. Followingis a description of a lensing system which can vary its focal distance.

[0030] Lensing System with Variable Focal Length

[0031] To vary the focal length, the lensing system 14 includes avariable focus lens (VFL). In some embodiments the VFL has its focusvaried by controlling the shape or thickness of the lens. In otherembodiment the VFL has its focus varied by varying the index ofrefraction of the lens. FIG. 5 shows an electro-mechanically variablefocus lens (VFL) 60 which changes its shape. A central portion 62 of theVFL 60 is constructed of a piezoelectric resonant crystalline quartz. Inoperation, a pair of transparent conductive electrodes 64 provide anelectrical field that deforms the piezoelectric material in a knownmanner. Such deformation changes the thickness of the central portion 62along its optical axis to effectively change the focus of the VFL 60.Because the VFL 60 is a resonant device, its focal length variesperiodically in a very predictable pattern. By controlling the time whena light pulse enters the resonant lens, the effective focal position ofthe VFL 60 can be controlled.

[0032] In some applications, it may be undesirable to selectively delaypulses of light according to the resonant frequency of the VFL 60. Insuch cases, the VFL 60 is designed to be nonresonant at the frequenciesof interest, yet fast enough to focus for each image pixel.

[0033] In an alternative embodiment, the variable focus lens is formedfrom a material that changes its index of refraction in response to anelectric field or other input. For example, the lens material may be anelectrooptic or acoustooptic material. In the preferred embodiment, thecentral portion 62 (see FIG. 11) is formed from lithium niobate, whichis both electrooptic and acoustooptic. The central portion 62 thusexhibits an index of refraction that depends upon an applied electricfield or acoustic energy. In operation, the electrodes 64 apply anelectric field to control the index of refraction of the lithium niobatecentral portion 62. In another embodiment a quartz lens includes atransparent indium tin oxide coating that forms the electrode 64.

[0034] In another embodiment shown in FIG. 6, a lens 70 includes acompressible cylindrical center 72 having a gradient index of refractionas a function of its radius. A cylindrical piezoelectric transducer 74forms an outer shell that surrounds the cylindrical center 72. When anelectric field is applied to the transducer 74, the transducer 74compresses the center 72. This compression deforms the center 72,thereby changing the gradient of the index of refraction. The changedgradient index changes the focal length of the center 72.

[0035] In another embodiment shown in FIG. 7 the variable focus elementis a semiconductor device 80 that has an index of refraction thatdepends upon the free carrier concentration in a transmissive region 82.Applying either a forward or reverse voltage to the device 80 through apair of electrodes 84 produces either a current that increases thefree-carrier concentration or a reverse bias that depletes the freecarrier concentration. Since the index of refraction depends upon thefree carrier concentration, the applied voltage can control the index ofrefraction. Memory 86 and control electronics 88 may be used to controlthe index of refraction.

[0036] In still another embodiment shown in FIG. 8 a plurality of lenses90-92 are cascaded in series. One or more piezoelectric positioners94-96 move one or more of the respective lenses 90-92 along the lightpath changing the focal distance of the light beam. By changing therelative position of the lenses to each other the curvature of the lightvaries.

[0037] According to one control approach, the lensing system 34continuously varies its focal length as needed to maintain a constantspot size. Referring to FIG. 9 the light distance detector 36 andlensing system 14 are coupled in a feedback loop. The output of thelight distance detector 36 is fed to focal control electronics 100. Thefocal control electronics 100 vary the focal length of a VFL 102 tomaintain a constant spot size (e.g., the prescribed standard spot sizepreviously described). The focal length at any given sample timecorrelates to the light distance (i.e., depth of view) for such sample.

[0038] According to another control approach, the lensing systemperforms a sweep of the focal length range of the VFL during each lightsample to be measured. During the sweep the spot 54 (see FIG. 4) willachieve its smallest size. The focal length at such time is used todefine the light distance.

[0039] According to these control techniques, the precise light distancefor any given sample is determined from the focal length of the lensingsystem 14 at the time such sample is taken. One of ordinary skill in theart will appreciate that a specific distance can be derived from thefocal length using the various optical parameters (e.g., magnificationfactors, relative positions of components) of a system 30 embodiment.

[0040] Scanning System

[0041] In one embodiment, the scanning system 32 includes a resonantscanner for performing horizontal scanning and a galvanometer forperforming vertical scanning. The scanner serving as the horizontalscanner receives a drive signal having a horizontal scanning frequency.Similarly, the galvanometer serving as the vertical scanner receives adrive signal having a vertical scanning frequency. Preferably, thehorizontal scanner has a resonant frequency corresponding to thehorizontal scanning frequency. In other embodiments the vertical scanneralso is a resonant scanner.

[0042] One embodiment of a resonant scanner is described in related U.S.patent application (Attorney Docket No. OT2.P17 filed on the same dayand having the same inventive entity) Ser. No. 09/188,993 filed Nov. 9,1998 of Michael Tidwell et al. for Scanned Beam Display With AdjustableAccommodation. The content of that application is incorporated herein byreference and made a part hereof. The resonant scanner includes a mirrordriven by a drive circuit (e.g., electromagnetic drive circuit orpiezoelectric actuator) to oscillate at a high frequency about an axisof rotation. The drive circuit moves the mirror responsive to a drivesignal which defines the frequency of motion.

[0043] Referring to FIG. 2, background light 12 impinges on the mirror39 of one scanner 38, then is reflected to another scanner 40, where itsmirror 41 deflects the light toward the lensing system 34. As thescanner mirrors 39, 41 move, different portions (e.g., pixel areas) ofthe background field of view are directed toward the lensing system 34and light distance detector 36.

[0044] In alternative embodiments, the scanning system 32 insteadincludes acousto-optical deflectors, electro-optical deflectors, orrotating polygons to perform the horizontal and vertical lightdeflection. In some embodiments, two of the same type of scanning deviceare used. In other embodiments different types of scanning devices areused for the horizontal scanner and the vertical scanner.

[0045] Image Capturing System

[0046] Referring to FIG. 10, an image capturing system 150 is shown inwhich image data is obtained and stored for each pixel within the fieldof view F for a single still frame or for multiple video image frames.The system 150 operates in the same manner as described for the system30 of FIG. 2 and like parts performing like functions are given the samepart numbers. In addition to detecting light distance however, lightintensity and light color also is detected for each pixel within thefield of view. Accordingly, a light intensity sensor 152 is includedalong with color sensor 154. One of ordinary skill in the art willappreciate that the sensors 152, 154 and 36 may be combined into acommon device, or that the color sensing and intensity sensing can beachieved with a common device. Further, rather than color detection grayscales may be detected for black and white monochromatic viewing.

[0047] For each pixel in the field of view, image data is obtained andstored in memory storage 156. The image data includes the pixelcoordinates, the determined light distance, the light intensity and thelight color. Such image data may be recalled and displayed at displaydevice 158 to replay the captured image frame(s). A controller 160coordinates the field of view scanning and the image replay.

[0048] Although preferred embodiments of the invention have beenillustrated and described, various alternatives, modifications andequivalents may be used. Therefore, the foregoing description should notbe taken as limiting the scope of the inventions which are defined bythe appended claims.

What is claimed is:
 1. A variable focus lens apparatus in an opticalscanning system, the apparatus comprising: a lens with an alterablephysical attribute; and means for dynamically altering said physicalattribute; wherein the altering means varies the focus of the lens toallow the optical scanning system to measure an optical distance of asample of light.
 2. The apparatus of claim 1, wherein the physicalattribute is one attribute from the group of physical attributesincluding: shape, thickness and index of refraction.
 3. The apparatus ofclaim 1, in which the physical attribute is shape, said altering meanscomprising means for altering the shape of the lens to achieve avariation in focus of the lens.
 4. The apparatus of claim 1, in whichthe physical attribute is thickness, said altering means comprisingmeans for altering the thickness of the lens to achieve a variation infocus of the lens.
 5. The apparatus of claim 1, in which the physicalattribute is index of refraction, said altering means comprising meansfor altering the index of refraction of the lens to achieve a variationin focus of the lens.
 6. A method of altering lens focus, comprising:generating a control signal; and dynamically altering a physicalattribute of a lens in response to the control signal, the physicalattribute from the group of physical attributes including, lensthickness and lens index of refraction, wherein by altering the physicalattribute, focus of the lens is varied.
 7. A method of altering lensfocus, comprising: generating a control signal; and dynamically alteringcurvature of a lens in response to the control signal, wherein byaltering the lens curvature, focus of the lens is varied.